We describe a dual-core 8051 microcontroller system featuring the synchronous and asynchronous (clockless) mode of operation. The synchronous mode of operation is achieved by means of a synchronous 8051 microcontroller core, while the asynchronous mode of operation is achieved by means of an asynchronous 8051 microcontroller core. The 8051 microcontroller system features shared embedded program and data memories that enable the switching between the two microcontroller cores during program execution. The measured energy, speed and electromagnetic interference of both microcontroller cores will be compared at different operation workloads.